1. Field of the Invention
The present invention relates to a diffraction grating, a laser apparatus, and a manufacturing method for the diffraction grating.
2. Description of the Related Art
Conventionally, there is a diffraction grating used in an ArF or Krf excimer laser apparatus as a reflection-type diffraction grating, which is used, for example, for dispersing a wavelength of far ultraviolet rays. Such a diffraction grating is used as a band-narrowing element and acts as a kind of a resonator by combining an output mirror in a discharge chamber. Here, the high proportion of light quantity that returns at a predetermined order upon the reflection, that is, high diffraction efficiency, is important for utilizing incident light without being wasted as far as possible, and for acquiring a preferable operation of the laser apparatus. Accordingly, the diffraction grating for the excimer laser apparatus commonly has an asymmetrical triangle shape for converging light at a specific order. Japanese Patent No. 4549019 discloses a diffraction grating for realizing an increase reflection by forming a protective layer on a reflective layer, in order to further enhance the diffraction efficiency. However, the diffraction efficiency depends on the grating shape of a single asymmetrical triangle grating, and therefore the grating shape must be suitable for the efficiency when trying to enhance the efficiency according to the condition of a film. Accordingly, U.S. Patent Publication No. 2010/0328775 discloses a method for enhancing the efficiency by performing the angle setting of interior angles of the triangle where the light is incident to a long side of the triangle, while the diffraction grating is commonly used in a condition in which the light is incident to only a short side of the asymmetrical triangle grating.
A surface (what is referred to as a “counter surface”) corresponding to a long side in the grating to be actually manufactured requires surface accuracy that is equivalent to a surface (what is referred to as a “blaze surface”) corresponding to the short side, in order to achieve a high efficiency by using the method disclosed in U.S. Patent Publication No. 2010/0328775. Here, a metal mold used when manufacturing (molding) the diffraction grating for the excimer laser apparatus is commonly formed by plastic working or by cutting. The plastic working has a principle that a deformation is performed by applying pressure, and therefore acquiring high accuracy for both the processed surfaces of the counter surface and the blaze surface is difficult. In the cutting, the counter surface is finished by a process of “rough path” that has a large cutting volume, and therefore the surface accuracy is not preferable. Additionally, when a finishing path is also applied to the counter surface in a manner similar to the blaze surface so as to reduce the cutting volume, the top of the blaze surface, which is connected to the counter surface to be finished, may collapse, causing an abnormality of the grating shape, and the result is not preferable because of causing a factor for lowering the efficiency. That is, while the diffraction grating may have a preferable shape in the design, the actual manufacturing is difficult.